The present invention relates to the electrodeposition of tin, lead and tin-lead alloys, and more particularly to aqueous baths for the electrodeposition of tin, lead or tin-lead alloys and methods for electrodepositing employing such baths. Still more particularly, the invention relates to reduced foaming baths for the electrodeposition of tin, lead or tin-lead alloys.
Coatings of tin and/or lead are desirable in the fabrication of a variety of electronic devices, circuits and connectors, such as printed circuits or integrated circuits, as protective layers, etch-resistant materials, stable surfaces for soldering, and the like.
Many aqueous baths for the electrodeposition of coatings of tin and/or lead are known and available in the art. Typical baths include aqueous acidic baths based upon fluoborate or fluosilicate electrolytes (see, e.g., Beckwith U.S. Pat. No. 3,769,182 et al. and Hsu U.S. Pat. No. 4,118,289 ). Of somewhat more recent vintage are aqueous acidic baths based upon alkanesulfonic acid or alkanolsulfonic acid electrolytes. See, e.g., Dohi et al., "Bright Solder And Indium Plating From Methane Sulfonic Acid", Proceedings of Electroplating Seminar, July, 1978; Dohi, et al., "Electrodeposition Of Bright Tin-Lead Alloys From Alkanolsulfonate Baths", Proceedings of Interfinish 80; Opaskar U.S. Pat. No. 4,582,576, et al.; Opaskar U.S. Pat. No. 4,662,999 et al.; Nobel U.S. Pat. Nos. 4,565,609; 4,565,610; 4,599,149; 4,617,097; and 4,701,244,, et al. The foregoing patents are incorporated herein by reference. Generally speaking, the baths based upon alkane- or alkanolsulfonic acids are preferred since baths containing fluoborates are corrosive and require care in handling and operation due to concerns regarding toxicity.
For obvious economic reasons, high-speed electroplating of tin, lead and tin-lead alloys is quite desirable. In seeking to achieve this goal, electroplating is desirably conducted at very high current densities. As a consequence, there occurs a fairly vigorous evolution of hydrogen gas at the plated surfaces in the form of fine bubbles, and these bubbles produce a dense foam on the surface of the electroplating bath, particularly in the presence of surfactants which are commonly employed in tin and/or lead baths to solubilize brightening agents and/or to attain improved smoothness and levelling of the metal deposit. This effect is attenuated by the common practice of establishing a vigorous circulation of the plating bath as a means for further increasing the speed of electroplating. Indeed, it is not uncommon to encounter dense foams rising several feet from the electrolyte in the collecting tank.
Excessive foam formation during electroplating can lead to non-uniform plating, increased consumption of plating bath materials, and increased drag-out of the plating solution. In situations where foam height is significant, risk exists that the foam will end up in ventilation systems, leading to obvious loss of material and corrosion of the exhaust systems.
To deal with foam formation at the surface of electroplating solutions it is not uncommon to spray a defoaming agent directly into the foam so as to reduce surface tension and break down the foam. The effects of such defoaming techniques are generally of only short duration, however, and it is therefore necessary to repeat this periodic spraying throughout the electroplating cycle, a time-consuming procedure at best. Moreover, because most defoaming agents are insoluble in water and in typical aqueous tin and/or lead electroplating baths, excessive incorporation of defoaming agent by spraying into the foam eventually results in a film of defoamer being formed on the bath surface, which film ends up on the surface of plated parts as they are removed from the bath. This film leaves noticeable, undesirable stains on the surface of bright deposits and, for tin and/or lead deposits, adversely affects the reflow of such deposits.
It would be advantageous if tin and/or lead electroplating baths could be formulated so as to be inherently low foaming even at high current densities and under vigorous agitation. One possible means for achieving this result is to choose surfactants (used, e.g., to solubilize or disperse brighteners in the bath and/or to promote levelling and smoothness) which are inherently low-foaming. See, e.g., Eckles U.S. Pat. No. 4,384,930. Typically, however, the low foaming ability of such surfactants is insufficient to prevent excessive foaming at conditions of high-speed plating. Direct incorporation into the bath of defoaming or foam inhibiting agents is also problematic due to the earlier-noted insolubility of such agents at plating bath conditions.